It is frequently desired to measure features of a repetitive signal. When the useful signal is embedded in random noise, a number of waveform records representative of respective repetitions of the repetitive signal may be stored, and an averaged waveform may be generated by computing the average of the stored waveform records. If the noise is random, the noise present in the averaged waveform is reduced with respect to the stored waveform records. Therefore, measurements made on the averaged waveform reveal more information about the useful signal than those made on the stored waveform records.
Waveform records are stored by digitizing the repetitive signal and writing the resulting sequence of N digital words per record into memory. A signal is digitized by sampling the signal at a constant clock rate, and quantizing each sample using an analog-to-digital converter. The averaged waveform is generated by averaging the ith words of the stored records for i=1 . . . N. However, if the clock pulses do not occur at precisely the same points within each repetition, the averaged waveform record will be distorted. For example, if only two records were averaged and each contained a ramp of uniform slope and of duration t from a lower voltage level V.sub.o to a higher voltage level V.sub.o +V, and the records were identical except that the second repetition was delayed such that the start of the ramp of the second repetition occurred at a time .DELTA.t, relative to a fixed reference point, later than the time at which the start of the ramp of the first repetition occurred, when the records are averaged the slope of the ramp in the averaged waveform is changed from V/t to V/(t+.DELTA.t). If the two stored waveform records were sinusoidal, the amplitude of the averaged waveform is dependent upon the jitter between the two records.
In the Tektronix 1980 Automatic Video Measurement Set, waveform records representative of respective repetitions of a repetitive segment of a television signal are stored and averaged, and features of the averaged record are measured, for example in order to determine whether the television signal complies with prescribed standards. Jitter among the records is removed using an algorithm known as ALIGN. The ALIGN algorithm is subject to the disadvantage that it does not operate properly if the spectral characteristics of the jitter are not uniform. Also, execution of the ALIGN algorithm is time consuming since it involves three fast-Fourier transformations.